Multimodal Diffuse Optical Imaging - Biomedical Optical Imaging Technologies: Design and Applications - page 350

Fig. 10.2 Absorption coefficient of major chromophores vs. optical wavelength

photons emitted. The lifetime is defined as the average amount of time the molecule
spends in the excited state following absorption of photons. Both parameters are
dependent (however weakly) on environmental factors such as temperature, solvent,
pH, etc. [ 5 ]. Numerous endogenous molecules fluoresce in the UV and visible
spectral range [ 6 ]. However, the NIR range is relatively free from background
autofluorescence and fluorescence applications for thick tissue imaging focus
mainly on exogenous fluorophores. Numerous fluorescent compounds have been
synthesized with appropriate spectral features for optical applications. Historically,
the first dye of interest was synthesized in 1871 by von Baeyer and is still the most
dominant fluorophore for clinical uses: fluorescein. Fluorescein is characterized by a
relatively high absorptivity (also known as extinction coefficient) and high quantum
yield. The dye exhibits a fluorescence peak around 520 nm. Thus, fluorescein has
found few applications in imaging deep-seated tissue. To overcome the limitation of
fluorescein, many fluorescent dyes with spectral features falling in the NIR window
have been synthesized with excellent fluorescent properties [ 87 ]. However, to date,
indocyanine green (ICG) is the only fluorescent contrast agent approved by the FDA
with optimal NIR spectral features. It is commonly used clinically for diagnostic
purposes [ 7 - 9 ] and has been used in optical mammography. The absorption spectra
of the main chromophores and ICG are provided in Fig. 10.2 .